A Jikany Nuer fighter in a village in Upper Nile, South Sudan. Less than a month after the cease-fire was signed, a rebel leader’s hometown was attacked.

Goran Tomasevic / Reuters

By NICHOLAS KULISH

February 12, 2014

NAIROBI, Kenya — The ranks of displaced people have swelled to nearly 900,000, close to a tenth of the entire population. Humanitarian groups warn that millions could go hungry if fields remain unplowed before the coming rainy season. Aid workers themselves are on the run, hiding ever deeper in the bush to escape attack.

Fighting has continued in South Sudan, both rebels and government officials say, in spite of the cease-fire agreement last month that was meant to bring peace to the young nation while a broader political solution was found.

Negotiators from the two sides will meet again in Ethiopia this week. But in a sign of the continuing hostilities, the hometown of Riek Machar, a former vice president and the leader of the rebels, has been attacked since the cease-fire went into force, with more than 1,100 homes there destroyed by fire, according the Satellite Sentinel Project, which monitors the region using satellite imagery.

Both the United States and the United Nations have voiced concern about the cease-fire violations. On Wednesday, the United Nations secretary general, Ban Ki-moon, expressed “deep concern” about reports of fighting and skirmishes in parts of Unity and Upper Nile States.

Thousands have been killed since the fighting began on Dec. 15 with a clash at a military barracks in Juba, South Sudan. President Salva Kiir accused his rival, Mr. Machar, of staging a coup attempt. Mr. Machar denied that there was a coup plot but forces loyal to him took up arms against the government.

“We are deeply concerned by reports of violations by both the government of South Sudan and antigovernment forces of the cessation of hostilities agreement,” Jen Psaki, a spokeswoman for the State Department, said in a statement last weekend. “We urge the redeployment or phased withdrawal of foreign forces invited by either side, and warn of the serious consequences which could result from any regionalization of this conflict.”

Negotiators for the two sides remain far apart in their demands. The rebels have demanded that foreign forces, in particular the Ugandan Army, which has fought alongside the South Sudanese military, leave the country before discussions can continue.

Secretary of State John Kerry said on Tuesday that he had spoken by phone with Mr. Kiir and Mr. Machar, as well as with the Ugandans and the Ethiopians, in an effort to find a solution.

“We also feel deeply committed, given past lessons, to try to prevent the chaos and the genocide that too often comes of the violence that can occur if things break down,” Mr. Kerry said. “We don’t want this to cascade into a more violent repetition of the past.”

About 724,000 people have been displaced within South Sudan by the fighting, while another 145,000 have fled to Uganda, Kenya, Sudan and Ethiopia. Nearly 75,000 people are being housed at United Nations bases, afraid of what could happen if they try to return home while the combatants are still skirmishing.

“Very, very few people have gone back home; most of the displaced people are still displaced or they have been traveling to neighboring countries,” said José Barahona, Oxfam’s country director for South Sudan. “This is not like flooding or an earthquake where the water’s gone down. The earth is not shaking.”

People’s shops have been looted or their homes burned to the ground. The livestock they rely on for survival have been taken from them. South Sudan is a poor country under the best circumstances, one where a large segment of the population lives on the edge of hunger. There are few paved roads or airstrips and once the rainy season begins it will become extremely difficult to move supplies to affected communities, much less for hundreds of thousands of people to return home.

The needs are already dire. So far, just 302,500 of the internally displaced — fewer than half of the total — have been reached by assistance. Humanitarian aid workers have been severely restricted in their work by the fighting, watching as combatants have commandeered their vehicles and looted their facilities.

Staff from Doctors Without Borders were forced to evacuate Leer, Mr. Machar’s hometown, seeking refuge in the bush. “The sporadic contact we are able to make with our staff paints a desperate picture for them and the unknown thousands living in terrible conditions, vulnerable to disease, dehydration, malnutrition and attack,” Raphael Gorgeu, the head of mission in South Sudan for Doctors Without Borders, said in a statement.

Those still in touch with the group “report that worsening security has pushed them further into the bush,” he said. “They have split into smaller groups to decrease the chance of attack and divided their supplies of medicines, which they are saving to treat only the most life-threatening cases.”

Brig. Gen. Lul Ruai Koang, military spokesman for the rebels, said in a telephone interview that there was “no strategic reason, there was no tactical reason to attack Leer.”

“It’s just a small town on the bank of the river,” he added. “The intention was to hurt, to kill relatives and people that are close to” Mr. Machar.

General Koang said that government forces deliberately killed livestock. “It was a well-planned action on Leer with immediate consequences and long-term consequences,” he said. “The immediate consequence is killing people. The long-term consequence is destroying their livelihood.”

Michael Makuei Lueth, South Sudan’s information minister, said in a telephone interview that it was Mr. Machar’s own forces that had attacked his hometown while it was under government control. “It is not actually peaceful because the rebels are not respecting the cessation of hostilities,” he said.

The lack of monitoring mechanisms were to blame for the continued attacks, he said. “There have been a lot of violations on the side of the rebels, so many of them,” said Mr. Lueth, citing assaults in Upper Nile, Jonglei and Unity states.

Skye Wheeler, a South Sudan researcher for Human Rights Watch, said that the United Nations should speed up its promised report on human-rights violations. Human Rights Watch has found evidence of mass arrests and the targeted killing of civilians, often on the basis of ethnicity.

“Regular and timely reporting is crucial in an environment like this and can help prevent violations from happening as well,” Ms. Wheeler said. “Evidence is already beginning to disappear. Every day matters at this point.”